Embodiments described herein relate to diesel engine exhaust systems. More particularly, embodiments described herein relate to precious metal based diesel oxidation catalysts for use in exhaust after-treatment systems of diesel engines that do not burn ultra low sulfur diesel (ULSD) fuel.
Sulfur occurs naturally in crude oil and sulfur removal is an increasingly important part of the diesel refining process. The petroleum industry is producing Ultra Low Sulfur Diesel (ULSD) fuel, a cleaner-burning diesel fuel containing a maximum of fifteen parts-per-million (ppm) sulfur. Many countries, including the United States, mandate the use of ULSD fuel. However, some countries do not require the use of ULSD fuel. In markets where ULSD fuel is not mandated, the sulfur content of diesel fuel can be hundreds or thousands of parts-per-million.
Precious metal catalysts are used to reduce emissions of harmful gasses from diesel engine exhaust. Environmental and human health concerns are the impetus for a major reduction in the sulfur content of diesel fuels and emission levels from diesel engines. In markets outside of North America, where ULSD fuel is not mandated, it is cumbersome and impractical to use precious metals in the exhaust after-treatment systems because sulfur deposits form on the active sites of the catalyst in the form of a sulfate or other sulfur-metal compound, which renders the catalyst ineffective, known as “sulfur poisoning”. Poisoning of the catalyst requires frequent servicing of the exhaust after-treatment to burn off the sulfur. For this reason, most diesel engine manufacturers in the non-low sulfur markets refrain from configuring the exhaust after-treatment systems with precious metal based diesel oxidation catalysts.